Viral transactivator genes generally encode transactivating factors or proteins which activate the expression of genes under the control of promoters which repond to the actions of the transactivation. This activation promotes viral replication. Viruses such as HTLV-III (HIV), Hepatitis B virus, and Herpes Simplex Virus have transactivator genes encoding transactivating factors which in trans promote viral replication. The viral transactivator for of HIV is known as TAT, the viral transactivator of Hepatitis B Virus is the X gene product and the viral transactivator for Herpes Simplex is designated ICP4. Chemical compounds or biological materials which interfere with the activity of transactivating factors are good candidates for antiviral therapeutics.
For example, the development of drugs to treat AIDS involves screening vast numbers of agents for anti-HIV activity. Anti-HIV activity in turn can be assessed by a variety of parameters.
For example, it is known that HIV requires several viral proteins for effective viral replication that include but are not limited to reverse transcriptase (RT), protease, transactivating protein (TAT), and REV protein. The switch from viral latency to active replication requires the regulatory gene products TAT and REV. TAT protein transactivates the HIV-LTR promoter and amplifies viral replication many thousand fold. The TAT- responsive sequence has been mapped within the LTR sequence. The REV gene product is involved in the transport of viral messenger RNAs from nuclei to cytoplasm and is required for the production of viral structural proteins coded for by GAG, POL, and ENV genes. When the REV gene is impaired the infected cells do not make the viral structural proteins and no mature virions are produced. An agent which inhibits TAT would be expected to arrest HIV in the latent stage and can therefore be used therapeutically for patients infected with HIV including AIDS and ARC patients as well as for asymptomatic carriers. Thus it is important to have screening assays which can effectively detect the anti-TAT activity of chemical compounds.
The Hepatitis B virus (HBV) X gene product transactivates the viral promoters and is required for the virus to propagate in animal hosts. [See Colgrove et al., J. Virol., 63: 4019 (1989)]. HBV causes persistent infections of liver cells that predispose hosts to liver cancer. An X gene inhibitor would suppress viral replication and thereby reduce the risk of liver cancer. The responsiveness of HBV promotors to X gene products has been demonstrated with heterologous gene constructs in laboratory tissue culture cell lines. [Colgrove, et al, J. Virol. 63: 4019 (1989)].
The Herpes Simplex virus (HSV) gene expression is coordinately regulated. The early and late genes (also called .beta. and .gamma. genes respectively) expression depend on the expression of the immediate early gene (also called .alpha. gene) expression. In particular the immediate early protein ICP4 transactivates the early and late gene promoters and is required for productive infection. [Maromara-Nazo et al., Virology 149: 152 (1986) and Gelman et al., P.N.A.S. 82: 5265 (1985)]. An inhibitor of ICP4 would block the reactivation of latent virus and would thereby have potential therapeutic applicability.
Screening assays which detect the anti-viral transactivating activity of various agents would be very useful in aiding the isolation of agents useful to treat various diseases.